50mm

I was listening to This Week in Photography and this listener question came up: "I have three daughters under four and I just bought an entry level DSLR (with the kit lens). Should my next purchase be a flash or a new lens?"

I formed my answer before I listened to Scott Bourne's thoughts. (Scott Bourne is the host and has been a pro photographer for longer than I've been alive.)

Turns out, we have the exact same thoughts on the matter, and I was pretty pleased with myself. (The answer was to buy a new, fast [wide aperture] fixed-focal length lens, like a 50mm, because it photographs little kids beautifully. Also little dogs.)